Telephony Glossary

The public switched telephone network (PSTN) is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators, providing infrastructure and services for public telecommunication.

https://en.wikipedia.org/wiki/Public_switched_telephone_network

A private branch exchange (PBX) is a telephone exchange or switching system that serves a private organization and performs concentration of central office lines or trunks and provides intercommunication between a large number of telephone stations in the organization.

https://en.wikipedia.org/wiki/Business_telephone_system#Private_branch_exchange

Integrated Services for Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.

https://en.wikipedia.org/wiki/Integrated_Services_Digital_Network

The Session Initiation Protocol (SIP) is a communications protocol for signaling and controlling multimedia communication sessions. The most common applications of SIP are in Internet telephony for voice and video calls, as well as instant messaging all over Internet Protocol (IP) networks.

https://en.wikipedia.org/wiki/Session_Initiation_Protocol